This invention relates to bonding of a cation exchange membrane and, more particularly, to the bonding of a cation exchange membrane and a fluorine-containing resin sheet which is to be installed in a chlor-alkali electrolyzer.
A fluorine-containing cation exchange membrane exhibits superior chemical resistance and mechanical strength when used as a diaphragm in the production of sodium hydroxide and chlorine from the electrolysis of brine by the ion exchange membrane process. This is especially true in cases of perfluorosulfonic acid type cation exchange membrane having sulfonic acid groups as the prevailing cation exchange group. The perfluorosulfonic acid type cation exchange membrane features low electrolytic voltage and tends to permeate hydroxide ions diffused from the cathode chamber to the anode chamber. This countermigration of hydroxide ions results in disadvantageously low current efficiency. To increase current efficiency, by resolving this weak point and to improve physical strength and, hence, dimensional stability, cation exchange membranes have been proposed in which a perfluorocarboxylic acid type ion exchange membrane is attached to the cathode side of the membrane, or the cathode side of the membrane is transformed into perfluorocarboxylic acid type by chemical treatment. A common practice for increasing strength of cation exchange membrane is to interpose mesh-state reinforcing material, such as a Teflon (Registered Trademark) mesh, between two layers of cation exchange membrane, or within the perfluorosulfonic acid type cation exchange membrane. Although the performance of this cation exchange membrane in brine electrolysis is excellent, the membrane is very costly. The need for a more economical solution is recognized by those skilled in the art.
An arrangement has been proposed, as shown in FIG. 1, in which the cation exchange membrane is shaped into a bag A. The anode B is positioned inside of the bag A. The cathode (not shown) is positioned outside the bag (Japanese Patent Laid Open Nos. 53-95899 and 53-106679). In this arrangement, however, the effective area of cation exchange membrane is composed only of the interleaved part by the anode and the cathode; the other remaining parts represent a non-contributive area to the electrolysis.
For this reason, for decreasing the membrane cost, discussions have suggested that only the part C, which is interleaved between the anode and the cathode, as shown in FIG. 2 and FIG. 3, be fabricated as the cation exchange membrane and that other parts D be fabricated from relatively low cost and highly chemical-resistant fluorine-containing resin.
However, an effective means has not been found for bonding the cation exchange membrane and fluorine-containing resin sheet directly to form a bag element composed of a cation exchange membrane and fluorine-containing resin sheet. Thus, the formation of a bag element as shown in FIG. 2 or FIG. 3, has in practice been impossible.
Further considerations are relevant to the fabrication of dual material bag elements. Before being installed in a chlor-alkali electrolyzer, a flat sheet of such cation exchange membrane may be shaped into bags, or films of it with small areas may be combined into a sheet of specified area and, in addition, damaged membranes must be repairable, by bonding.
In order to bond two partially overlapping ion exchange membranes at the overlapping part of heat bonding, it is necessary for sulfonic acid sides E and carboxylic acid sides F, respectively, to be bonded so that they have an identical direction, as shown in FIG. 4, and a sulfonic acid side E and carboxylic acid side F of the adjacent cation exchange membranes C are to be in contact with each other.
However, even if heat is applied with compression in the state as shown in FIG. 4, both cation exchange membranes C cannot be bonded due to the small affinity between carboxylic acid and sulfonic acid, or if bonded, the tensile strength to be attained will be extremely low. In contrast, sulfonic acid group has relatively strong affinity with sulfonic acid group and a bonded membrane with satisfactory strength can be obtained from partially overlapped sulfonic acid type cation exchange membranes.